Method and installation for the continuous preparation of pellets for the production of snack type food products

ABSTRACT

The invention relates to a method for the continuous preparation of pellets intended for the production of crusty and honeycombed snack-type food products from a natural starchy raw material having a starch content that is equal to or greater than 60% and a protein content below 13%. According to the invention, the raw material is preconditioned; said raw material is fed continuously into an extrusion machine with two co-rotary and co-penetrating screws; a supply and transport step, a cooking step, a degassing step and a cooling step are performed in the extrusion machine; a continuous bead of cooked dough having a moisture content of between 20 and 30% is formed; the dough is oriented in a vertical direction and pushed through a multiple-outlet mould to form several continuous beads; the beads are cut transversely to form pellets. The pellets are subsequently dried to produce pellets having a moisture content of between 10 and 12%. The invention also relates to an installation for the continuous preparation of pellets.

[0001] The present invention relates to a process and an installationfor the continuous production of pellets that are intended for themanufacture of snack-type food products with a cellular and crunchytexture from a starchy raw material having a starch content equal to orgreater than 60% and a protein content of less than 13%.

[0002] The invention also relates to a food product in the form ofexpandable pellets that can be obtained by such a production process andare intended for the manufacture of snacks with a cellular and crunchytexture.

[0003] The pellets are semifinished products produced from a starchy rawmaterial and having a low water content of between 10 and 20% and a highbulk density. These products have a gelatinized and glassy appearance.

[0004] These pellets are intended to be subsequently expanded, forexample in the presence of hot air at a temperature of around 250° C. orin an oil bath at a temperature of around 180° C., and once expandedthey are assimilated into snacks. The pellets have a lifetime of morethan one year and, for the user, they permit easier production control.

[0005] The production of pellets intended for the manufacture ofsnack-type food products comprises a succession of several steps, thesebeing:

[0006] preconditioning the raw material by adding water and/or steam inorder to obtain a mixture possessing a moisture content of between 15and 40%;

[0007] cooking the mixture at a temperature of between 90 and 150° C.,allowing the starches to be converted and gelatinized in order to obtaina dough possessing a moisture content of between 15 and 40%;

[0008] densifying and shaping the dough;

[0009] individually cutting the products;

[0010] drying the products by reducing their moisture to a content ofaround 11%;

[0011] resting the product in order to obtain a uniform distribution ofwater within the pellet; and

[0012] expanding the pellets, for example in the presence of hot air orin an oil bath.

[0013] Hitherto, in the conventional processes, the cooking step iscarried out in a first extruder having a single screw or two co-rotatingscrews and provided with a cylindrical die in order to form severalcontinuous strands of dough that are cut so as to obtain a product inthe form of beads several millimeters in diameter.

[0014] The beads are then sent into a second extruder having a singlescrew, which makes it possible to compress the particles and to obtain ahomogeneous dough, which leaves this second extruder in the form of acontinuous extrudate.

[0015] This extrudate is transferred directly to a mold in order to formseveral strands that are cut into pellets of the desired shape.

[0016] However, this process is a two-step process that uses a firstextruder for cooking and a second extruder for forming, consequentlyinvolving means for transferring the product between the two extrudersand means for maintaining its temperature, thereby increasing the pelletmanufacturing time, which is around 4 to 8 minutes.

[0017] In addition, the startup time of the installation is relativelylong, of the order of one hour, given the number of extruders that makeup this installation.

[0018] Finally, the raw material used in the known processes is astarchy raw material having at least one modified and/or pregelatinizedpart, which is an expensive raw material.

[0019] The object of the invention is to avoid these drawbacks byproposing a process and an installation for the continuous production ofpellets that are intended for the manufacture of snack-type foodproducts, without a charging break in the manufacturing line andallowing a product of excellent quality to be obtained, while reducingthe manufacturing time and the consumption of energy.

[0020] The subject of the invention is therefore a process for thecontinuous production of pellets that are intended for the manufactureof snack-type food products with a cellular and crunchy texture from anatural starchy raw material having a starch content equal to or greaterthan 60% and a protein content of less than 13%, characterized in that:

[0021] the material is preconditioned by adding water and/or steam inorder to bring the moisture content of said raw material to between 15and 40%;

[0022] the material is continuously introduced into a twin-screwextruder having intermeshing co-rotating screws rotated at a speed ofgreater than 200 rpm about parallel axes inside a barrel, the residencetime of the material in the extruder being between 30 seconds and 1 min30 s;

[0023] the following steps are carried out in succession in the barrelof the extruder:

[0024] a step of feeding and transporting the raw material,

[0025] a step of cooking this raw material at a temperature of between90 and 150° C. and with a residence time of said material equal to orgreater than 25 seconds, said cooking step comprising between two andfive mixing steps in order to form a cooked dough having a moisturecontent of between 24 and 35% and

[0026] a step of venting the dough and a step of pumping and cooling itto a temperature of between 80 and 100° C.;

[0027] a continuous dough extrudate having a moisture content of between20 and 30% is formed at the exit of the extruder;

[0028] the dough extrudate is oriented into a vertical direction so asto flow downward through an orienting chamber and at the same time theflow of the dough is regulated;

[0029] the dough is introduced into a spreading chamber and, on leavingthis chamber, the dough is pushed through a multi-outlet mold in orderto form a multitude of continuous strands having the desired shape;

[0030] said strands are cut transversely in order to form the pelletswith a moisture content of between 20 and 30%; and

[0031] the pellets are progressively dried at a temperature of between50 and 110° C. and a relative humidity of between 40 and 80% for morethan three hours in order to obtain pellets having a moisture content ofbetween 10 and 12%.

[0032] The subject of the invention is also an installation for thecontinuous production of pellets that are intended for the manufactureof snack-type food products with a cellular and crunchy texture from anatural starchy raw material having a starch content equal to or greaterthan 60% and a protein content of less than 13%, characterized in thatit comprises:

[0033] a preconditioner for mixing water and/or steam with the rawmaterial in order to bring the moisture content of said raw material tobetween 15 and 40%;

[0034] an extruder formed from two intermeshing co-rotating screwsrotated at a speed of greater than 200 rpm about parallel axes inside anelongate barrel provided with intersecting bores, the length of thescrews being between 32 and 48 times the diameter of said screws andsaid screws defining, from the upstream end to the downstream end:

[0035] a zone for the continuous feeding into the barrel of thepreconditioned raw material and for transporting this raw material,

[0036] a zone for cooking this raw material at a temperature of between90 and 150° C. and with a residence time of said raw material equal toor greater than 25 seconds, the length of the screws in the cooking zonebeing between 18 and 30 times the diameter of one of said screws andthis cooking zone comprising between two and five mixing screw segmentsin order to obtain a cooked dough having a moisture content of between24 and 35%,

[0037] a zone for venting the cooked dough and a zone for pumping andcooling it to a temperature of between 80 and 100° C. and

[0038] a zone for discharging the dough from the extruder in order toform a continuous dough extrudate having a moisture content of between20 and 30%;

[0039] a chamber for orienting the cooked dough extrudate into avertical direction so as to flow downward and for regulating the flow ofthe dough;

[0040] a chamber for spreading the cooked dough, provided at its exitwith a multi-outlet mold in order to form a number of continuous strandshaving the desired shape;

[0041] means for cutting said strands transversely in order to form thepellets; and

[0042] means for progressively drying the pellets at a temperature ofbetween 50 and 110° C. and a relative humidity of between 40 and 80% formore than three hours in order to obtain pellets having a moisturecontent of between 10 and 12%.

[0043] The subject of the invention is also a food product in the formof expandable pellets that are intended for the manufacture of snacks,characterized in that it is obtained by the abovementioned productionprocess.

[0044] According to other features of the invention, the product isproduced from a natural starchy raw material having a starch contentequal to or greater than 60% and a protein content of less than 13%, thestarchy raw material being based on cereals and/or tubers.

[0045] Other features and advantages of the invention will appear in thecourse of the following description, given with reference to theappended drawings in which:

[0046]FIG. 1 is a schematic side view of an installation for thecontinuous production of food products in the form of pellets, accordingto the invention;

[0047]FIG. 2 is a sectional view in a vertical plane passing through theaxis of a screw of an extruder of the installation according to theinvention;

[0048]FIG. 3 is a longitudinal sectional view of the extruder;

[0049]FIG. 4 is a cross-sectional view on the line 4-4 of FIG. 3;

[0050]FIG. 5 is a schematic perspective view of one portion of thescrews in the cooking and mixing zone of the extruder; and

[0051]FIG. 6 is a schematic sectional view of the dough orienting anddistributing chambers and of the cutting means of the installationaccording to the invention.

[0052]FIG. 1 shows schematically an installation for the continuousproduction of pellets that are intended for the manufacture ofsnack-type food products with a cellular and crunchy texture from anatural starchy raw material whose moisture content is between 9 and14%. This raw material preferably has a starch content equal to orgreater than 60% and a protein content of less than 13%.

[0053] The starchy raw material that can be used within the context ofthe invention may be a material based on cereals, such as, for example,a wheat, corn, rice, barley or oat flour, or a mixture of these flours,or else this material may be a flour based on tubers, such as, forexample, manioc, potato or a mixture of these products. This starchy rawmaterial may contain up to 100% native starch and its particle size isbetween 50 and 800 microns.

[0054] The term “native starch” is understood to mean a starch that hasundergone no physical and/or chemical pretreatment and can be usedwithout any modification.

[0055] Generally speaking, the installation denoted in its entirety bythe reference 1 comprises:

[0056] a preconditioner 10 fed with raw material via a metering device2;

[0057] an extruder, denoted by the general reference 20, which, as willbe seen later, is formed with two intermeshing co-rotating screws;

[0058] a chamber 40 for orienting the dough extrudate output by theextruder 20;

[0059] a chamber 50 for spreading the dough in a multi-outlet mold 55;

[0060] transverse cutting means 60 for forming the pellets; and

[0061] pellet drying means 80 for obtaining pellets having a moisturecontent of between 10 and 12%.

[0062] The installation may also include means (not shown) for expandingthese pellets in the presence of hot air at a temperature of about 250°C. or in an oil bath at a temperature of around 180° C. in order toobtain snacks having a moisture content of less than 3%.

[0063] The preconditioner is formed by an enclosure 11 provided, at itsupstream end, with a port 12 for introducing the raw material into theenclosure 11 by means of the metering device 2 and, at its downstreamend, with a port 13 for outputting the raw material, the moisturecontent of which has been raised to a value of between 15 and 40%. To dothis, the enclosure 11 of the preconditioner 10 is connected to waterand/or steam supply means, consisting for example of a pump 14, and theraw material is mixed in the enclosure 11 with water and/or with steamand transported to the output orifice 13 by means of two shafts providedwith adjustable blades and denoted by the reference 15. Salt or sugarmay also be introduced with the raw material into the enclosure 11 ofthe preconditioner.

[0064] The starchy raw material thus moistened and/or more or lessgelatinized is introduced into the extruder 20 by means of a feed hopper16 and this extruder is of the intermeshing co-rotating twin-screw type,as shown in FIGS. 2 and 3.

[0065] The extruder 20 has two screws 21 and 22 rotated about their axesby a motor and a reduction gear (these not being shown) inside anelongate enclosure forming a barrel 23 that envelopes them.

[0066] The screws 21 and 22 are provided in particular with helicalflights or with segments for the treatment of the raw materialintroduced into the barrel 23, as will be seen later, which screws meshwith each other, and the internal wall of said barrel 23 forms twointersecting cylindrical lobes of inside diameter slightly greater thanthe outside diameter of the flights and of the treatment segments.

[0067] The two screws 21 and 22 are driven at the same rotation speedand in the same direction in such a way that these two screws areidentical, the flights and the treatment segments being simply offsetwith respect to one another.

[0068] As in particular FIG. 4 shows, the screws 21 and 22advantageously consist of splined shafts, 24 and 25 respectively, onwhich the screw portions are stacked.

[0069] The internal bore of these screw portions is provided withgrooves corresponding to the splines on the shaft, and the external partis provided with helical flights or with segments for treating thematerial, the pitch and the configuration of which differ depending onthe portion in question, for treating and transporting this material.

[0070] Thus, it is possible to use a large number of portions havingconfigurations that differ according to the type of treatment to becarried out on the material.

[0071] The extruder 20 shown in FIGS. 2 and 3 include a zone A forcontinuously feeding the premoistened starchy raw material into thebarrel 23 and for transporting this material toward the downstream ofsaid extruder 20.

[0072] In this zone A, the barrel 23 is pierced, at its upstream endrelative to the direction of flow of the material, by a port 26surmounted by a hopper 16 into which the raw material delivered by thepreconditioner 10 is continuously poured.

[0073] In the feeding zone A, the screws 21 and 22 are provided withlong-pitch threads 28 so as to transport the raw material introduced viathe orifice 26 which opens widely onto the two screws 21 and 22, so asto distribute said material into the flights of the screws.

[0074] In the embodiment shown in the figures, the barrel 23 includes,at the start of the feeding and transporting zone A, at least one waterinjection port 27 connected to a feed pump 3 in order to adjust themoisture content of the dough according to the moisture content of thedough to be obtained at the exit of the extruder 20, which must bebetween 20 and 27%. In this zone A, because of the mixing of thematerial by the flights 28, the temperature of the materialprogressively rises up to a temperature of around 130° C.

[0075] Thus, the material is transported toward the downstream of theextruder 20 into a zone B for mixing and cooking this material, which isheated to a temperature of between 90 and 150° C. in order to obtain adough at the exit of this zone B.

[0076] Depending on the type of raw material, the cooking zone Bcomprises between two and five mixing screw segments.

[0077] In the embodiment shown in FIGS. 2 and 3, the mixing and cookingzone B is composed of three identical portions B1 placed one afteranother.

[0078] Each portion B1 comprises, on the one hand, a first part 30provided with helical flights 30 a that mesh with one another and, onthe other hand, a second part 31 provided with diamond-shaped bilobatemixing segments, as shown in FIG. 5. The tops of these bilobate segments31 a are truncated in order to provide a controlled passage for thematerial, and these bilobate segments of each screw are offset by 90°with respect to one another. In addition, the bilobate segments 31 a ofthe two screws 21 and 22 mesh with one another.

[0079] The length of each bilobate segment 31 a is approximately equalto the diameter of one of the screws, 21 or 22.

[0080] Thus, the material leaving the first zone A undergoes, in thefirst part 30 of the first portion B1, compression by the flights 30 aand then this material undergoes, in the second part 31 of said portionB1, controlled shearing by the bilobate segments 31 a.

[0081] In this way, the material undergoes in the zone B a controlledsuccession of compression and shearing and, over the entire length ofthis zone B, its temperature is raised from 90 to 150° C., allowing acooked dough to be obtained at the exit of this zone B.

[0082] To progressively raise the temperature of the material up toabout 150° C., the barrel 23 of the extruder 20 is equipped with heatingmeans.

[0083] As shown in FIGS. 2 and 3, the heating means are formed by acircuit 32 for circulating a heat-transfer fluid, or by resistanceheating elements placed around the barrel and extending over the entirelength of the zone B.

[0084] These heating means may also be formed by an induction heatingsystem.

[0085] The length of the screws 21 and 22 in the mixing and cooking zoneB is between 18 and 30 times the diameter of one of said screws. Theresidence time of the material in the cooking zone B is equal to orgreater than 25 seconds and the moisture content of the dough leavingthis zone is between 24 and 35%.

[0086] According to an alternative embodiment, and especially in thecase of cereals, the zone B may include, after the last portion B1, atleast one additional portion comprising a first part provided withconventional helical flights that mesh with one another and a secondpart formed from a shearing segment consisting of counterflights ofknown type.

[0087] On leaving this zone B, the dough is transferred into a ventingzone C in which the screws 21 and 22 are provided with helical flights34 and in which the barrel 23 is pierced by a port 35.

[0088] Upon being transferred into the zone C, the cooked doughundergoes venting, either natural venting or accelerated venting, and inthe latter case the port 35 is connected to a vacuum pump 36, as shownin FIG. 1. This venting therefore allows water in vapor form containedin the dough to be extracted and results in cooling of this dough.

[0089] After this venting zone C, the extruder 20 has a zone D forpumping the dough and for cooling it to a temperature of about 90° C. atthe exit of this extruder 20.

[0090] In this zone D, the screws 21 and 22 of the extruder are providedwith helical flights 37 and the barrel 23 has cooling means that areformed, for example, by a circuit 38, provided within said barrel 23,for circulating a coolant.

[0091] The dough is thus pumped by the flights 37 of the screws 21 and22 and transferred into a zone E for forming a continuous doughextrudate. To do this, the barrel 23 has an outlet 39 for the dough, themoisture content of which is between 20 and 30%.

[0092] The length of the screws 21 and 22 in the zones C and D isbetween 8 and 16 times the diameter of one of said screws.

[0093] The residence time of the material in the extruder is between 30seconds and 1 min 30 s and the rotation speed of the screws is greaterthan 200 rpm and preferably greater than 250 rpm if this raw materialconsists of cereals.

[0094] At the end of the zone D, the barrel 23 may have a port (notshown) for injecting coloring agents and/or flavorings into the rawmaterial.

[0095] As shown in FIGS. 1 and 6, the dough that leaves the extruder 20at a temperature of around 90° C. is transferred in succession into theorienting chamber 40 and into the spreading chamber 50. The orientingchamber 40 may be provided with means for injecting coloring agentsand/or flavorings into the dough. These chambers 40 and 50 are providedwith means of known type (not shown) for regulating the temperature ofthe dough to between 80 and 100° C.

[0096] The orienting chamber 40 is formed by an elbowed pipe 41 fororienting the dough extrudate leaving the extruder 20 into a verticaldirection, i.e. perpendicular to the axis of the extruder 20, so that itflows downward.

[0097] Preferably, the orienting chamber 40 is provided with means forregulating the distribution of the dough, which means are formed, in theembodiment shown in FIG. 6, by a first screen 42 placed at the inlet ofthis orienting chamber 40 and by a second screen 43 placed at the exitof said orienting chamber 40.

[0098] According to an alternative embodiment, means for regulating thedistribution of the dough may be formed by a screen 42 placed at theinlet of this chamber 41 or by a screen 43 placed at the outlet of saidchamber 41.

[0099] The screen or screens 42 and 43 are placed perpendicular to thedirection of flow of the dough and make it possible to regulate thedistribution of the dough, which is pushed out of the extruder 20; thisdough, after it has passed into the orienting chamber 40, is transferredto the spreading chamber 50.

[0100] This spreading chamber 50 comprises a downwardy flared outer cone51 and a male inner cone, leaving between them a space for spreading thedough. This chamber 50 also includes, above the mold 55, a filter 53 forretaining particles larger than 800 microns that are contained in thedough and a premolding screen 54 for adjusting the dough draw ratesthrough the outlets of the mold 55.

[0101] Conventionally, this mold includes interchangeable inserts (notshown) that determine the shape of the product. Thus, the dough isspread out in the chamber 50 before passing in succession through thefilter 53, the premolding screen 54 and the mold 55 in order to form amultitude of continuous strands having the desired shape.

[0102] On leaving the mold 55, the strands run into a box 61 thatcontains the means 60 for cutting these strands transversely in order toform the pellets.

[0103] These cutting means 60 are formed, for example, by at least oneflexible and non-stick blade 62 that is rotated by a shaft 63 connectedto a gearmotor (not shown). This blade 62 lies in a plane extendingperpendicular to the direction in which the strands of dough leaving themold 55 run.

[0104] The pellets thus cut drop onto a screen 64 and are discharged viaan outlet pipe 65 by means of an air blowing system (not shown), the airbeing temperature- and flow-rate-regulated, and introduced into the box61 via a pipe 66. The temperature-regulated air introduced into the box61 via the pipe 66 passes through the screen 64 and drives the pelletsout of said box 61 via the pipe 65, and these pellets are transferred todrying stations and then to stations for packaging them in hermeticallysealed packages.

[0105] According to a preferred embodiment, the installation includesmeans 70 for discharging the dough output by the extruder 20, said meansbeing placed before the spreading chamber 50.

[0106] As shown in FIG. 6, these discharge means 70 are formed by abranch pipe 71 that runs into the orienting chamber 40 and is providedwith a flap 72 that can move between a position in which the branch pipe71 is closed off, as shown in solid lines in FIG. 6, in order to directthe dough extrudate toward the spreading chamber 50, and a position inwhich the pipe 41 of said orienting chamber 40 is closed off, as shownin dotted lines in this figure, in order to direct the dough extrudatetoward the discharge pipe 71.

[0107] Thus, until the water content of the dough output by the extruder20 has reached a predetermined value, generally between 20 and 30%, thatis to say upon starting up this extruder 20, the flap 72 is tilted byconventional-type drive means (not shown) in the position for closingoff the pipe 41 of the spreading chamber 40, so as to direct the doughinto the pipe 71 and consequently prevent this dough from passingthrough the spreading chamber 50 and contaminating the cutting assembly.As soon as the moisture content of the dough reaches the predeterminedvalue, the flap 72 is returned to its initial position for closing offthe discharge pipe 71, so as to direct this dough toward the spreadingchamber 50.

[0108] Thanks to this device, the startup time of the installation isshorter, around 10-20 minutes, whereas in conventional installations itis about one hour. This time saving is also due to the fact that theinstallation comprises only a single extruder.

[0109] The rotation speed of the screws 21 and 22 of the extruder 20 isgreater than 200 rpm depending on the raw material treated and theprofile of these screws 21 and 22 is adapted in order to achieveprogressive and controlled mixing, shearing and compression of thematerial in the extruder.

[0110] A wide variety of raw materials based on cereals and/or tubersmay be used, by themselves or as a mixture.

[0111] The pellet-type food product obtained by the process according tothe invention contains:

[0112] 40 to 99% by weight of starch-containing material;

[0113] 0 to 30% by weight of native or modified starch;

[0114] 0 to 60% by weight of other, non-starchy ingredients; and

[0115] 0 to 5% by weight of salt.

[0116] If the total percentage starch content in the formulation is lessthan or equal to 60%, the subsequent expansion of the end product isless and gives a firm and crunchy texture. Conversely, if the percentagestarch content is greater than 60%, the expansion is greater and thetexture more aerated and soft.

[0117] The installation also includes means 80 for drying the pellets.,by progressively drying them from 50 to 110° C. at a relative humidityof between 40 and 80% for more than three hours in order to obtain thesnacks having a uniform moisture content of between 10 and 12%.

[0118] Next, the pellets are expanded in order to obtain snacks with acellular and crunchy texture having a moisture content of less than orequal to 3%.

[0119] The installation according to the invention therefore allowscontinuous production of pellets intended for the manufacture ofsnack-type food products with a cellular and crunchy texture from anatural starchy raw material, the various formulations of which aregiven in the table below. FORMULATION TYPE OF INGREDIENT % 1 Corn flour98 Salt 2 2 Wheat flour 98 Salt 2 3 Potato starch 88 Potato granules orflakes 10 Salt 2 4 Corn flour 93 Corn starch 5.5 Salt 1 Emulsifier 0.5 5Corn starch 50 Wheat starch 25.5 Potato starch 12 Manioc starch 12Emulsifier 0.5

[0120] As an example, the parameters used in the installation accordingto the invention for formulation No. 1 were

[0121] amount of material entering the preconditioner: 200 kg/h

[0122] moisture content of the raw material in the preconditioner: 11.5%

[0123] rotation speed of the shafts of the preconditioner: 150 rpm

[0124] moisture content of the material leaving the preconditioner: 23%

[0125] temperature of the material leaving the preconditioner; 75° C.

[0126] screw speed of the extruder; 290 rpm

[0127] moisture content of the material in the feeding zone before themixing and cooking zone: 29%

[0128] temperature of the material in the feeding zone before the mixingand cooking zone: 75° C.

[0129] temperature of the material in the mixing and cooking zone: 130°C.

[0130] temperature of the dough in the venting zone: 110° C.

[0131] temperature of the dough in the pumping and cooling zone: 95° C.

[0132] moisture content of the dough in the pumping and cooling zone:27%

[0133] temperature of the cut and formed pellet leaving the mold: 90-95°C.

[0134] moisture content of the cut and formed pellet leaving the mold:21%

[0135] rate of drawing of the cut and formed 85 pellet leaving the mold:cm/min

[0136] mean residence time of the material from entering the extruderuntil leaving the mold: 60-90 s

[0137] The pellets thus manufactured and then dried were semifinishedproducts with a low water content of generally between 10 and 12% andwith a high bulk density of around 200 to 700 g/l. They had agelatinized and glassy appearance and were intended to be expanded at alater stage.

[0138] These products had a long shelf life because of their lowmoisture content.

1. A process for the continuous production of pellets that are intendedfor the manufacture of snack-type food products with a cellular andcrunchy texture from a natural starchy raw material having a starchcontent equal to or greater than 60% and a protein content of less than13%, characterized in that: the material is preconditioned by addingwater and/or steam in order to bring the moisture content of said rawmaterial to between 15 and 40%; the material is continuously introducedinto a twin-screw extruder (20) having intermeshing co-rotating screws(21, 22) rotated at a speed of greater than 200 rpm about parallel axesinside a barrel (23), the residence time of the material in the extruderbeing between 30 seconds and 1 min 30 s; the following steps are carriedout in succession in the barrel (23) of the extruder (20): a step offeeding and transporting the raw material, a step of cooking this rawmaterial at a temperature of between 90 and 150° C. and with a residencetime of said material equal to or greater than 25 seconds, said cookingstep comprising between two and five mixing steps in order to form adough having a moisture content of between 24 and 35% and a step ofventing the dough and a step of pumping and cooling it to a temperatureof between 80 and 100° C.; a continuous dough extrudate having amoisture content of between 20 and 30% is formed at the exit of theextruder (20); the dough extrudate is oriented into a vertical directionso as to flow downward through an orienting chamber (40) and at the sametime the flow of the dough is regulated; the dough is introduced into aspreading chamber (50) and, on leaving this chamber, the dough is pushedthrough a multi-outlet mold in order to form a multitude of continuousstrands having the desired shape; said strands are cut transversely inorder to form the pellets with a moisture content of between 20 and 30%;and the pellets are progressively dried at a temperature of between 50and 110° C. and a relative humidity of between 40 and 80% for more thanthree hours in order to obtain pellets having a moisture content ofbetween 10 and 12%.
 2. The process as claimed in claim 1, characterizedin that, after the pellets have been dried, they are expanded in thepresence of hot air at a temperature of about 250° C. or in an oil bathat a temperature of around 180° C.
 3. The process as claimed in claim 1or 2, characterized in that the starchy raw material is based on cerealsand/or tubers.
 4. The process as claimed in any one of claims 1 to 3,characterized in that the starchy raw material has a particle size ofbetween 50 and 800 microns.
 5. The process as claimed in claim 1,characterized in that, at the start of the feeding and transportingstep, water is injected into the extruder (20) in order to adjust themoisture content of the dough according to the moisture content of thedough to be obtained at the exit of the extruder (20).
 6. The process asclaimed in any one of claims 1 to 5, characterized in that, for astarchy raw material consisting of cereals, the cooking step includes atleast one shearing step.
 7. The process as claimed in any one of claims1 to 6, characterized in that, for a starchy raw material consisting ofcereals, the rotation speed of the screws (21, 22) is greater than 250rpm.
 8. The process as claimed in any one of claims 1 to 7,characterized in that flavorings and/or coloring agents are injectedduring the pumping and cooling step.
 9. The process as claimed in anyone of claims 1 to 8, characterized in that flavorings and/or coloringagents are injected into the orienting chamber (40).
 10. The process asclaimed in any one of the preceding claims, characterized in that,between the exit of the extruder (20) and the molding, the temperatureof the dough is regulated to between 80 and 100° C.
 11. The process asclaimed in any one of the preceding claims, characterized in that,before the step of molding the dough, particles larger than 800 micronscontained in this dough are retained and a premolding operation iscarried out on said dough.
 12. The process as claimed in any one of thepreceding claims, characterized in that, during the startup phase of theextruder (20), the dough is directed to the exit of said extruder (20)into a discharge pipe (71) until the moment when the moisture content ofthis dough stabilizes at a value of between 20 and 30%.
 13. Aninstallation for the continuous production of pellets that are intendedfor the manufacture of snack-type food products with a cellular andcrunchy texture from a natural starchy raw material having a starchcontent equal to or greater than 60% and a protein content of less than13%, characterized in that it comprises: a preconditioner (10) formixing water and/or steam with the raw material in order to bring themoisture content of said raw material to between 15 and 40%; an extruder(20) formed from two intermeshing co-rotating screws rotated at a speedof greater than 200 rpm about parallel axes inside an elongate barrelprovided with intersecting bores, the length of the screws being between32 and 48 times the diameter of said screws and said screws (21, 22)defining, from the upstream end to the downstream end: a zone A for thecontinuous feeding into the barrel (23) of the preconditioned rawmaterial and for transporting this raw material, a zone B for cookingthis raw material at a temperature of between 90 and 150° C. and with aresidence time of said raw material equal to or greater than 25 seconds,the length of the screws (21, 22) in the cooking zone being between 18and 29 times the diameter of one of said screws and this cooking zonecomprising between two and seven mixing screw segments (31) in order toobtain a cooked dough having a moisture content of between 24 and 35%, azone C for venting the cooked dough and a zone D for pumping and coolingit to a temperature of between 80 and 100° C. and a zone E fordischarging the cooked dough from the extruder (20) in order to form acontinuous dough extrudate having a moisture content of between 20 and30%; a chamber (40) for orienting the cooked dough extrudate into avertical direction so as to flow downward and for regulating the fluidof the dough; a chamber (50) for spreading the cooked dough, provided atits exit with a multi-outlet mold (55) in order to form a number ofcontinuous strands having the desired shape; means (60) for cutting saidstrands transversely in order to form the pellets; and means forprogressively drying the pellets at a temperature of between 50 and 110°C. and a relative humidity of between 40 and 80% for more than threehours in order to obtain the snacks having a content of between 10 and12%.
 14. The installation as claimed in claim 13, characterized in thatthe barrel (23) of the extruder (20) includes, at the start of thefeeding and transporting zone A, at least one port (27) for injectingwater in order to adjust the moisture content of the dough according tothe moisture content of said dough to be obtained at the exit of thisextruder (20).
 15. The installation as claimed in claim 14,characterized in that each mixing screw segment (31) of the cooking zoneB is formed by a diamond-shaped bilobate segment (31 a), the tops of thelobes of which are truncated in order to provide a controlled passagefor the material, the bilobate segments (31 a) of each screw (21, 22)being offset by 90° with respect to one another and said bilobatesegments (31 a) of the two screws (21, 22) meshing with one another. 16.The installation as claimed in claim 15, characterized in that thelength of each bilobate segment (31 a) is approximately equal to thediameter of one of said screws (21, 22).
 17. The installation as claimedin any one of claims 13 to 16, characterized in that the cooking zone Bincludes at least one shearing segment formed by counterflights in thecase of a starchy raw material consisting of cereals.
 18. Theinstallation as claimed in any one of claims 13 to 17, characterized inthat the length of the screws (21, 22) in the venting, pumping andcooling zones C and D is between 8 and 16 times the diameter of one ofsaid screws.
 19. The installation as claimed in any one of claims 13 to18, characterized in that the barrel (23) of the extruder (20) includes,in the pumping and cooling zone D, at least one port for injectingcoloring agents and/or flavorings.
 20. The installation as claimed inany one of claims 13 to 18, characterized in that the orienting chamber(40) includes means for injecting coloring agents and/or flavorings intothe dough.
 21. The installation as claimed in claim 13, characterized inthat the orienting chamber (40) and the spreading chamber (50) includemeans for regulating the temperature of the dough to between 80 and 100°C.
 22. The installation as claimed in claim 13 or 21, characterized inthat the orienting chamber (40) comprises an elbowed pipe (41) providedwith means (42, 43) for regulating the distribution of the dough. 23.The installation as claimed in claim 22, characterized in that the meansfor regulating the distribution of the dough are formed by a screen (42)placed at the inlet of the orienting chamber (40) and/or a screen (43)placed at the exit of said chamber (40), said screen or screensextending perpendicularly to the direction of flow of the doughextrudate.
 24. The installation as claimed in claims 13 to 21,characterized in that the spreading chamber (50) comprises a downwardlyflared outer cone (51) and a male inner cone (52), leaving between thema space for spreading the dough.
 25. The installation as claimed inclaim 13 or 24, characterized in that the regulating chamber (50)includes, above the mold (55), a filter (53) for retaining particleslarger than 800 microns that are contained in the dough and a premoldingscreen (54) for said dough.
 26. The installation as claimed in claim 13,characterized in that the cutting means (60) are formed by at least onerotating blade (62) extending perpendicular to the direction of movementof the strands, said blade (62) being placed in a box (61) connected toan adjustable air blowing system for discharging the pellets from thisbox (61).
 27. The installation as claimed in any one of claims 13 to 16,characterized in that it includes means (70) for discharging the doughoutput by the extruder (20) before the spreading chamber (50) until themoment when the moisture content of this dough reaches a value ofbetween 20 and 30%.
 28. The installation as claimed in claim 27,characterized in that the discharge means (70) are formed by a branchpipe (71) that runs into the orienting chamber (40) and is provided witha flap (72) that can move between a position in which the branch pipe(71) is closed off, in order to direct the dough extrudate toward thespreading chamber (50), and a position in which the pipe (41) of theorienting chamber (40) is closed off, in order to direct the doughextrudate toward said discharge pipe (71).
 29. A food product in theform of expandable pellets that are intended for the manufacture ofsnacks, characterized in that it is obtained by the production processas claimed in any one of claims 1 to
 12. 30. The food product as claimedin claim 29, characterized in that it is produced from a natural starchyraw material having a starch content equal to or greater than 60% and aprotein content of less than 13%.
 31. The food product as claimed inclaim 30, characterized in that the starchy raw material is based oncereals and/or tubers.